The present invention relates to a stepping motor with a magnetic sensorxe2x80x94this stepping motor is widely used in information devices and the likexe2x80x94and more particularly, it relates to a mounting mechanism of the sensor.
Recently, stepping motors are extensively used in various fields, such as information devices and audio equipment including, a printer, facsimile, image scanner, copying machine, laser beam printer, CD-ROM, DVC. Other fields including factory automation (FA) equipment such as machine tools, automotive components, and a field of home appliances also employs a large number of stepping motors. This extensive use of the stepper motors is mainly thanks to a low cost, and simple operation realizing a speed control or a positioning control.
Nowadays, a stepping motor coming with a sensor is widely available in order to improve the performance thereof.
A sensor mounting mechanism to the stepping motor is disclosed by the Japanese Patent Application Non-Examined Publication No. H09-253139 and Japanese Utility Model Application Non-Examined Publication No. S62-135558.
FIG. 10 and FIG. 11 illustrate the construction of conventional stepping motors coming with a sensor.
FIG. 10 is a cross section illustrating a structure of a conventional stepping motor with a magnetic sensor.
In FIG. 10, driving coil 51 of the stepping motor is covered by yoke 52, which is a comb-teeth-like pair of magnetic substance. Rotor 53 is disposed to face the inner wall of yoke 52 with a given space. Rotor 53 comprises output shaft 54, magnet 55 which is magnetized to N and S polarities in pairs, and a resin-made coupler for coupling shaft 54 with magnet 55. Magnetic sensor 56 is soldered to printed circuit board 57 attached independently to the stepping motor. With a given space, sensor 56 faces to a face-to-face type magnetic drum 58 mounted to shaft 54 and having pairs of polarities. The revolving of rotor 53 rotates drum 58. At this time, a voltage corresponding to the magnetic force of the pairs of N and S polarities of drum 58 is tapped off from sensor 56 through a circuit printed on board 57. Sensor 56 and drum 58 are, in general, protected by sensor cover 59 mechanically as well as electrically.
FIG. 11 is a cross section illustrating a structure of a conventional stepping motor with a photo sensor.
In FIG. 11, slit-disc 73 having a plurality of fine slits is disposed on output shaft 72 of rotor 71. Photo sensor 74 either one of transparent or reflective type detects the slits on disc 73, and outputs signals corresponding to the sensed result cyclically. Sensor 74 and disc 73 are, in general, protected by sensor cover 75 electrically and mechanically.
This sensor is generally called optical encoder, and used in various fields. The sensor features (a) easy mounting, (b) a large output signal, and (c) versatile output signals available depending on a slit-type on the slit-disc. However, it is difficult to adjust positioning between the slit-disc and rotor for obtaining an output signal from the sensor corresponding to the rotor. The sensor thus tends to produce dispersion in performance, and the optical encoder is generally expensive.
As such, the conventional sensor has both advantages and disadvantages in mounting thereof, a magnitude of an output, position adjustment, a resolution and a cost.
Both in FIG. 10 and FIG. 11, functions of sensing rotation of the output shaft are the same; however, in FIG. 10, a magnetic sensor may directly detects a polarity position of the magnet fixed to the rotor. Since the magnetic drum uses magnets, the magnet polarities of magnetic drum and those of rotor can be aligned depending on a way of magnetizing.
On the other hand, in FIG. 11, the slit-disc and the magnet polarities should be aligned at an independent process. Adjustable range of dimensions (or angle) decreases at a higher sensing resolution of the slit-disc, therefore, this alignment is substantially difficult, and adversely affects the cost and stableness in performance.
A problem common to these two instances is great influence of heat to the sensor because the stepping motor generates large amount of heat by itself. When a motor is controlled, a sensor signal should be stable, and influence by heat to the circuits around and a control IC should be also considered. In a conventional structure, only the sensor has been placed at a remote spot from the motor; however, this is not enough for the heat problem. When a sensing function and controlling function of the sensor are to be attached to a stepping motor or other motors generating great amount of heat by themselves, the sensor block should be positively kept away from the motor in order to avoid the heat influence to the sensor and the controlling IC. This structure needs compactness and efficiency in assembling as well.
In the conventional stepping motor having a general-use sensor as discussed above, it is difficult to keep a given space between the magnet and sensor and decrease heat influence to the sensor as well as to the control IC.
The present invention addresses the problems discussed above and aims to provide a new mounting mechanism of a sensor for improving a positioning accuracy of the sensor mounting position as well as decreasing influence by heat generated by a stepping motor to the sensor and a controller.
The stepping motor of the present invention comprises the following elements:
(a) a rotor driven by exciting a driving coil;
(b) a position detector including a magnetic flux generator for outputting a signal in the same phase as a polarity position of a magnet mounted to the rotor and a magnetic sensor receiving magnetic force from the magnetic flux generator;
(c) a controller for controlling a stepping motor responsive to a position detection signal from the position detector;
(d) a board on which the magnetic sensor and the controller are mounted; and
(e) a sensor cover for positioning the magnetic sensor, dissipating heat from the controller and covering at least parts of the position detector as well as the controller.
The stepping motor can be divided into two independent sub-assemblies, i.e. a motor sub-assembly including the driving coil, rotor, magnetic flux generator, and a sensor sub-assembly including the board and sensor cover.
This structure decreases degradation such as lowering an output to the sensor or the controller due to the heat generated by the stepping motor, and allows an assembly process to be divided into the sensor sub-assembly and the motor sub-assembly. As a result, the present invention contributes to improving the productivity as well as lowering material loss.